Polymerisation process

ABSTRACT

The present invention relates to a polymerisation process, in particular to the polymerisation of olefins in a reactor system comprising two reactors in series, and most particularly provides a process for the polymerisation of monomer in at least first and second reactors operated in series, which process comprises contacting a first stream comprising vapour derived from the effluent withdrawn from the second reactor with a feed stream to the second reactor, said feed stream comprising effluent derived from the first reactor.

The present invention relates to a polymerisation process, and inparticular to the polymerisation of olefins in a reactor systemcomprising two reactors in series.

The production of polymer powder by polymerisation reactions of monomersin the presence of catalysts is well-known. For example, processes areknown and widely operated commercially using both fluidised bed reactorsand slurry phase reactors.

In a slurry polymerisation process the polymerisation is conducted in astirred tank or, preferably, a continuous loop reactor in which a slurryof polymer particles in a liquid medium comprising hydrocarbon diluentis circulated. During the course of polymerisation, fresh polymer isgenerated by the catalytic polymerisation of monomer and polymer productis removed from the reactor by removing a portion of the slurry.

In a single reactor system the slurry withdrawn from the reactor istreated to separate the polymer particles from the hydrocarbon diluentand other components, such as unreacted monomers, which it is generallydesired are recycled to the reactor.

The process where polymer is formed in two reactors in series is alsoknown. The separate reactors can be operated to produce the same productin each reactor, but are most advantageously operated to producedifferent products in each reactor, in particular to make bimodalpolymer products.

In such a process, polymer is produced in the first reactor, withdrawnin the form of a slurry and passed to a second reactor where furtherproduction of polymer takes place. Polymer slurry withdrawn from thesubsequent reactor is treated to separate the polymer solids fromdiluent and unreacted reactants, which it is generally desired torecycle to the process.

In a typical separations process, which is also generally used forsingle reactor systems, withdrawn slurry is heated and passed to a firstseparation step in which the majority of the diluent and unreactedmonomers and comonomers are separated from the polymer solids as a gas(flash gas) at relatively high pressure such that the gas can becondensed without compression and recycled. This is commonly referred toas a “flash step”.

Remaining solids and residual diluent are then sent to a secondseparation step, which may be a further flash tank or may be a flushcolumn where the solids are contacted with a flush gas, such asnitrogen, to remove residual diluent, monomers and comonomers. Thesecond separation step is usually at a lower pressure, and diluent,unreacted monomer and any comonomer separated in the second separatorneed to be separated from any flush gas, and usually need to becompressed prior to recycle.

Thus, the overall polymerisation process generally includes both highpressure and low pressure recovery systems for recovery and recycle ofdiluent, monomers and comonomers.

The polymer solids may be taken to further processing, such as blendingor pelleting, or to storage.

WO 2006/015807 discloses a polymerisation process in which the gasstream recovered from flashing diluent and unreacted monomer from aslurry of polymer solids is passed to a fractionator. The application ofthe fractionator to a process comprising two polymerisation loopreactors operated in series is also described. In particular withrespect to FIG. 3 there is described a process where the fractionator isused to treat both the flash gas from the second reactor in series andalso gas separated from the effluent between the two reactors.

We have now found an improved process for the operation of apolymerisation reactor where gas recovered from the effluent exiting areactor is contacted with a feed stream to the same reactor.

Thus, in a first aspect the present invention provides a process for thepolymerisation of monomer in at least first and second reactors operatedin series, which process comprises contacting a first stream comprisingvapour derived from the effluent withdrawn from the second or asubsequent reactor with a feed stream to the second reactor, said feedstream comprising effluent derived from the first reactor.

The contacting can take place in any suitable contacting vessel.

The feed stream is generally in a liquid form, and most preferably is aslurry of polymer solids in a liquid medium from the first reactor.

The first reactor is generally the previous reactor in the series to thesecond reactor.

The first stream comprises vapour derived from the effluent withdrawnfrom the second reactor or a subsequent reactor. The “subsequentreactor” option can apply where there are 3 or more reactors present inseries, at least one of these being subsequent to the second reactor.

For example, three reactors present in a series may be considered asreactors “A”, “B” and “C”. The present invention may be applied bycontacting a feed stream to reactor B, which comprises effluent fromreactor A, and the first stream may comprise vapour derived from theeffluent withdrawn from reactor B or may comprise vapour derived fromthe effluent withdrawn from reactor C (or both).

Alternatively, or additionally, the present invention may be applied insuch a three reactor series between reactors B and C by contacting afeed stream to reactor C, which comprises effluent from reactor B, witha first stream comprising vapour derived from the effluent withdrawnfrom reactor C (i.e. in this option reactor B may be considered as the“first reactor” and reactor C as the “second reactor” of the presentinvention.)

Preferably, the feed stream to the second reactor is contacted with afirst stream comprising vapour derived from the effluent withdrawn fromthe second reactor, and the present invention will hereinafter beillustrated with respect to this option, although it will be equallyclear that the process could be equally applied with a first streamderived from a subsequent reactor where one is present.

As an example, a slurry polymerisation process produces an effluent froma second reactor in the form of a slurry of polymer solids in a liquidmedium comprising diluent. This stream is usually treated to vaporisethe components of the liquid medium, which are then separated from thepolymer solids. The first stream is preferably at least a portion ofthis separated stream, more preferably a majority of, yet morepreferably at least 80% by weight of, and most preferably essentiallyall of, this separated stream.

It is preferred that the first stream is passed from its source, forexample a flash tank or other means to separate vaporised medium fromthe polymer solids/rest of the effluent, to the contacting step withoutany compression.

It is generally preferred that a majority of, yet more preferably atleast 80% by weight of, and most preferably essentially all of the firststream is in vapour form when brought into contact with the feedstreamin this first aspect.

In particular, the heat applied to vaporise the liquid medium in theeffluent withdrawn from the second reactor and which is thereforepresent as heat in the first stream can be efficiently utilised tominimise or avoid any additional heat input requirement in thecontactor. In contrast, if the first stream is cooled significantlyprior to the contactor, as would be required for significant amounts ofvapour to condense, then some of that useable heat is lost. In somecircumstances it may then be necessary to apply additional heat in thecontactor or to the feed stream. It is therefore preferred that coolingof the first stream prior to entry into the contactor is avoided or atleast minimised.

More preferably, the first stream is passed directly from its source,for example a flash tank or other means to separate vaporised mediumfrom the polymer solids/rest of the effluent, to the contacting stepwith the feed stream to the second reactor. By “directly” is meantwithout intermediate treatment, such as direct or indirect cooling ortreatment to separate certain components, such as fines.

In a second aspect, the present invention provides a process for thepolymerisation of monomer in a reactor, which process comprisescontacting a first stream comprising vapour derived from the effluentwithdrawn from the reactor with a feed stream to the reactor, said firststream comprising a majority of the vaporised components of the liquidmedium in the effluent withdrawn from the reactor and at least 80% byweight of the first stream is in vapour form when brought into contactwith the feed stream.

The feed stream in this second aspect may be any stream comprising oneor more components to be fed to the reactor. The feed stream isgenerally in a liquid form. Most preferably the feed stream in thissecond aspect comprises effluent derived from a previous reactor inseries, in which case this second aspect is a preferred embodiment ofthe first aspect.

More generally, and whilst it is preferred that cooling of the firststream prior to entry into the contactor is avoided or at leastminimised in the first and second aspects, nevertheless, a portion ofthe first stream may condense prior to contact with the feedstream. Itis also possible that the first stream can contain small quantities ofsolids. For example, whilst the vaporised diluent should be separatedfrom the bulk of the polymer solids prior to being used as the firststream, it is possible that the vapour can contain entrained solids,generally referred to as “fines”. A particular advantage of the presentinvention is that these entrained solids do not need to be removed fromthe vaporised diluent/first stream prior to the contacting of the firststream with the feed stream to the second reactor, as is describedfurther below.

The pressure at which the first stream is separated from the polymerwithdrawn from the second reactor is preferably equal to or higher thanthe pressure at which it is brought into contact with the feedstream tothe second reactor. Thus, the stream may be passed to the contactingstep without compression.

By the process of the present invention at least part of the firststream may be recycled to the second reactor without compression and/orat least part of the first stream may be recycled to the second reactorwithout indirect cooling.

“Indirect cooling” as used herein means use of a cooling medium wherethe medium to be cooled and the cooling medium do not physically mix. Asused herein “indirect cooling” requires the deliberate use of a coolingmedium and excludes more general loss of heat to the surroundings frompipework and other equipment. Pipework and equipment may be lagged toreduce such heat losses.

Usually indirect cooling is applied using a cooling medium through thewalls of a pipe or vessel, such as in a heat exchanger. At least part ofthe first stream is preferably recycled to the second reactor as aliquid without any compression or indirect cooling.

More particularly, at least 10 wt % of the first stream passed to thecontacting step can be recycled to the second reactor in thepost-contacted feedstream to the second reactor. This is describedfurther below.

The first stream is preferably separated from the polymer withdrawn fromthe second reactor at high pressure, typically between 0.7 MPa and 1.5MPa, and preferably between 0.8 MPa and 1.2 MPa. The first stream ispreferably brought into contact with the feedstream to the secondreactor at high pressure, typically between 0.7 MPa and 1.5 MPa, andpreferably between 0.8 MPa and 1.2 MPa. Preferably, the pressure atwhich the first stream is brought into contact with the feedstream tothe second reactor is approximately the same as the pressure at whichthe first stream is separated from the polymer withdrawn from the secondreactor. However, a small pressure differential is usually present dueto inherent pressure drops in the connecting pipework.

The contacting generally results in post-contacted vapour and apost-contacted feedstream, the vapour being passed to further processingand the feedstream being passed to the reactor.

Generally, the present invention provides a process for thepolymerisation of monomer in at least two reactors operated in series,which process comprises contacting the first stream comprising vapourderived from the effluent withdrawn from the second reactor with thefeed stream to the second reactor to produce a second stream comprisingvapour which is passed to further processing and a third stream which ispassed to the second reactor.

As with the feed stream, the third stream is generally in a liquid formand more preferably comprises a slurry of polymer solids in a liquidmedium. The monomer in the process of the present invention ispreferably an olefin monomer. For avoidance of any doubt, the “monomer”as used herein refers to the olefin which is present in the largestamount in the formed polymer, and may also be referred to as the“principal monomer”, whilst the term “comonomer” as used herein refersto olefins other than the monomer which may be present. More than onecomonomer may be present.

The monomer is preferably ethylene or propylene, most preferablyethylene. Where ethylene is the monomer, propylene may be the comonomer,but the comonomer is preferably selected from 1-butene, 1-hexene and1-octene, with 1-hexene being most preferred.

Where propylene is the monomer, the comonomer is preferably selectedfrom ethylene, 1- butene, 1-hexene and 1-octene.

The comonomer is preferably 1-hexene.

Preferred diluents which may be used are inert hydrocarbons, morepreferably butanes, especially iso-butane, pentanes and mixturesthereof. Iso-butane is most preferred.

The present invention may be applied to a process for the polymerisationof monomer in two reactors connected in series.

Most preferably, the at least two reactors are slurry looppolymerisation reactors which produce effluents comprising a slurry ofpolymer solids (“polymer slurry”). In this embodiment, a polymer slurryis withdrawn as an effluent from the second reactor, and the firststream is derived from the effluent by flashing all or a portion of theliquid medium to form a vapour, separating this from the polymer solids,and using at least a portion of the separated vapour as the first streamas noted previously.

The polymer slurry withdrawn from the first reactor and contacted withthe first stream generally comprises solid polymer and a liquid mediumcomprising diluent and unreacted monomer. The first stream derived fromthe effluent withdrawn from the second reactor generally comprisesdiluent and monomer, and may also comprise comonomers, hydrogen andimpurities, such as alkanes and nitrogen.

In more detail, the present invention provides, as a third aspect, aprocess for the polymerisation of monomer in two slurry loop reactorsconnected in series, which process comprises:

-   -   1) Polymerising in a first loop reactor, monomer in the presence        of a diluent to produce a first polymer slurry comprising        polymer solids suspended in a liquid medium comprising diluent        and unreacted monomer,    -   2) Withdrawing a portion of the first polymer slurry as a first        effluent comprising solid polymer, diluent and unreacted        monomer, and passing said first effluent to a contactor to form        a second effluent comprising solid polymer, diluent and        unreacted monomer,    -   3) Passing said second effluent to a second loop reactor,    -   4) Polymerising in the second loop reactor, monomer in the        presence of said second effluent, to produce a second polymer        slurry comprising polymer solids suspended in a liquid medium        comprising diluent and unreacted monomer,    -   5) Withdrawing a portion of the second polymer slurry as a third        effluent comprising solid polymer, diluent and unreacted        monomer, and    -   6) Passing said third effluent to a separation step for        separating a first stream comprising vaporised diluent and        unreacted monomer from said polymer solids,        characterised in that at least a portion of the first stream is        passed to the contactor wherein it is contacted with the first        effluent to produce said second effluent and a second stream        comprising vaporised diluent and unreacted monomer.

The present invention has advantageously been found to result intransfer of components from the first stream into the feedstream for thesecond reactor. At the same time, components in the feedstream can betransferred into the first stream.

An example is entrained solids in the first stream. Entrained solids inthe first stream are preferentially entrained into the stream to thereactor, and thereby are recycled to the reactor. Such solids can becatalytically active and their recycle to the reactor not only preventsthe loss of such active components, but also prevents them being able toreact and potentially cause fouling in downstream processing of thevapour. Filters and/or cyclones, which are often used to removeentrained fines from the vapour, for example of flash tank overheadlines, can be avoided.

The ability to avoid filters operating on the first stream isparticularly advantageous. In particular, such filters can be prone toplugging due to condensation of components in the first stream at highpressure. Back-flushing of filters to remove blockages on high pressurefilters can be generally harder and has greater propensity to causefilter damage. Thus, filters on high pressure streams can beparticularly prone to operational problems.

Preferably, therefore, the present invention comprises no filters orsolids removal equipment which act on the first stream, and inparticular it is preferred that the first stream is passed from itssource, for example a flash tank or other means to separate vaporisedmedium from the polymer solids/rest of the effluent, to the contactingstep without passing through any filters. Further, the ability of thecontacting step to remove entrained fines from the first stream prior tosubsequent treatment of the first stream means that filters can beavoided in other parts of the high pressure recovery system. Moregenerally therefore the polymerisation process preferably comprises nofilters operating at pressures above 0.5 MPa, more preferably no filtersoperating above 0.4 MPa, and most preferably no filters operating above0.2 MPa.

The present invention can also remove components from the effluentstream from a first reactor passed to a second reactor which are notrequired or are required in lesser amounts in the second reactor than inthe first, and to enhance in the effluent stream passed to the secondreactor components which are required or are required in greater amountsin the second reactor than in the first. In particular, it has beenfound that contacting of the separated vapour from the slurry exitingthe second reactor, which can be inherently lower in the components notrequired or required in lesser amounts in the second reactor and higherin the components which are required or are required in higher amountsin the second reactor, results in a more favourable intermediatetreatment.

In addition to solids, where present in the first stream, as notedabove, the invention may be illustrated with respect to hydrogen andcomonomer components. In some bimodal processes operating in two loopreactors in series, hydrogen may be desired in the first reactor but notin the second, or at least the amount of hydrogen desired in the secondreactor is lower than in the first reactor. Similarly, comonomer may bedesired in the second reactor but not in the first reactor, or at leastthe amount of comonomer required is higher in the second reactor than inthe first reactor. Thus, the first polymer slurry and the first effluentalso comprise hydrogen, and the second polymer slurry and the thirdeffluent also comprise unreacted comonomer. In “conventional treatment”,the first effluent may be passed to an intermediate treatment step inwhich at least a portion of the hydrogen is separated from the firsteffluent prior to its passage to the second reactor. Whilst this canreduce the hydrogen significantly it can be difficult to removesufficient hydrogen in this way without a significant reduction inpressure and/or without significant diluent or monomer loss with thehydrogen (which may be passed to flare).

In the process of the present invention it has been found thatcontacting of the first stream/separated vapour from the slurry exitingthe second reactor, which has relatively reduced hydrogen and relativelyincreased comonomer compared to the feed stream/first effluent, resultsin an increase in the hydrogen in the second stream compared to thefirst stream and an increase in comonomer in the third stream/secondeffluent compared to the feed stream/first effluent.

The “increase” usually, and preferably, manifests itself as an increasein the absolute quantities of said components in said streams, measuredin mass flow rates of the particular components.

The “increase” usually in addition manifests itself as increase in theratios of the particular components to monomer in said streams, forexample hydrogen to ethylene ratio and comonomer to ethylene ratio maybe increased. In an alternative embodiment the increase may manifestitself only as an increase in the ratios of particular components tomonomer i.e. without a requirement for an increase in the absolute massflow rates. Thus, an increase in hydrogen is a particular stream meansthat the ratio of hydrogen to ethylene increases, whilst an increase incomonomer in a particular stream means that the ratio of comonomer toethylene increases.

It will be apparent that in order to maintain a mass balance in thepolymerisation system of the third aspect of the invention the majorityof the diluent and unreacted monomers recovered from the third effluentactually have to be recycled to the first reactor rather than thesecond. By the process of the present invention the second stream, in anopposite result to the second effluent passing to the second reactor, isenhanced in the components desired in the first reactor and poorer inthe components not desired, or desired in lesser quantities. Thus, inthe example of hydrogen and comonomer given previously, the secondstream is enhanced in hydrogen and poorer in comonomer than the firststream. This also therefore reduces the subsequent treatment of thevapour which is required prior to recycling to the first reactor.

More particularly, greater than 80%, such as greater than 90%, forexample essentially all (by which is meant greater than 99%) of thecomonomer in first stream prior to the contacting may be returned tosecond reactor via the feedstream to the second reactor from thecontacting.

In contrast, greater than 80%, such as greater than 90%, for exampleessentially all (by which is meant greater than 99%) of the hydrogen inthe feed stream to the second reactor prior to the contacting is removedfrom the feed stream and passed to further processing.

For the above reasons, the present invention is particularly applicableto processes for producing bimodal polymer products. The separation stepof the third aspect of the present invention is preferably a highpressure separation step, for example in a high pressure recoverysystem.

In a slurry polymerisation process the pressure and temperature in thehigh pressure recovery system are generally selected such that themajority of the diluent, monomer and comonomer are recovered in thevapour and can be condensed without compression for recycle to thereaction. Examples of such systems can be found, for example, in WO2005/003188 which discloses the use of a higher pressure flash stagefollowed by a lower pressure flush stage. However, processes are alsoknown where the lower pressure stage is a flash stage rather than aflush stage, or where both flashing and flushing occur in a singlestage. (It can be noted that a flush stage can also be referred to as a“purge stage”. The term “flush” is used herein for such steps to avoidany confusion with process purges, which are steps whereby streams areremoved from a polymerisation process, for example to flare. The term“purge” as used herein therefore refers to a stream which is removedfrom the process rather than a flush step.)

The terms “high pressure” and “low pressure” are used herein to indicaterelative pressures of different recovery systems.

Generally, however, “high pressure” as used herein generally refers tostreams and stages which are at a pressure of 0.5 MPa (5 bar) and above,and usually 0.7 MPa (7 bar) and above, and “low pressure” generallyrefers to streams and stages which are at a pressure of less than 0.5MPa (5 bar), usually less than 0.4 MPa (4 bar).

A low pressure recovery system, in contrast to the high pressurerecovery system, leads to recovered components, such as diluent, monomerand comonomer, at lower pressures, and which generally need compressionprior to recycle.

In a preferred embodiment the contacting of the first stream and thefeed stream take place in a contactor which is configured in associationwith a fractionator to yet further enhance the separation achieved. Inparticular, the second stream recovered from the contactor is passed toa fractionator, from which heavier components, including comonomer, arerecovered from the base and passed back to the contactor, whilst lightercomponents, such as hydrogen and monomer, and also diluent, arerecovered from the fractionator as a vapour for further treatment andrecycle.

Further, in a fourth aspect of the present invention there is provided aprocess for the polymerisation of monomer in a reactor, which processcomprises contacting a first stream comprising vapour derived from theeffluent withdrawn from the reactor with a feed stream to the reactor toproduce a second stream comprising vapour which is passed to furtherprocessing and a third stream which is passed to the reactor, saidcontacting taking place by

-   -   a. passing said first stream and said feed stream to a        contactor,    -   b. passing the second stream recovered from the contactor to a        fractionator,    -   c. recovering heavier components, including comonomer, from the        base of the fractionator and passing said components back to the        contactor,    -   d. recovering lighter components from the fractionator as a        vapour for further treatment and recycle.

The feed stream in this fourth aspect may be any stream comprising oneor more components to be fed to the reactor. The feed stream isgenerally in a liquid form. Most preferably the feed stream in thisfourth aspect comprises effluent derived from a previous reactor inseries, in which case this fourth aspect is a preferred embodiment ofthe first and third aspects of the present invention.

For example, with respect to the third aspect of the present invention,the contactor and fractionator are configured such that:

-   -   (i) slurry recovered from the contactor is recovered as the        second effluent and passed to the second reactor,    -   (ii) vapour recovered from the contactor is passed to the        fractionator,    -   (iii) liquid recovered from the fractionator is passed to the        contactor, and    -   (iv) vapour recovered from the fractionator is recovered.

More generally, the combined contactor/fractionator may be considered asa fractionation system which efficiently acts to fractionate the mixtureof feed stream/first effluent and first stream initially passed to thecontactor.

The vapour, or at least a portion thereof, recovered from the contactoror the contactor/fractionator combination is typically passed to one ormore steps which may include removal of inert components, especiallyinert light components such as nitrogen and ethane which can otherwisebuild-up in the system, and/or which may include removal of “heavy”components, such as comonomer and components heavier than comonomer,prior to recycle.

After the high pressure separation step for separating a vapourcomprising diluent and unreacted monomer from polymer solids the polymersolids are usually passed from the high pressure recovery system to alow pressure recovery system. The low pressure recovery system maycomprise a low pressure separation step for separating further diluent,unreacted monomer and unreacted comonomer from said solids, and arecycle system for recycling at least a portion of the further diluent,unreacted monomer and unreacted comonomer.

The low pressure recovery system/recycle system may comprise a lightsseparator and/or a heavies separator for removing such components fromthe process.

By “lights separator” as used herein is meant a separator which isoperated to provide separation of “lights” other than monomer, such ashydrogen and methane, from monomer and heavier compounds. As used herein“lights” means propane and molecules having a molecular weight less thanpropane.

By “heavies separator” as used herein is meant a separator which isoperated to separate compounds heavier than the comonomer.

The general concept of “lights” and “heavies” separators for separationof such components in polymerisation processes is well-known. Oneexample of such a system is taught by U.S. Pat. No. 6,292,191.

In a further aspect of the present invention it has surprisingly beenfound that hydrogen can be efficiently removed from the polymerisationsystem by treating two separate portions of the vapour stream recoveredfrom the contactor at low temperature but at different pressures.

Thus, in a further aspect, the present invention provides a process fortreatment of a first stream comprising vapour derived from the effluentwithdrawn from a polymerisation reactor, which process comprises:

-   -   i) contacting said first stream with a feedstream to the reactor        to produce a vapour comprising hydrogen,    -   ii) treating said vapour comprising hydrogen to produce and        separate from each other a first liquid stream and a third        stream comprising vapour,    -   iii) passing said third stream to a first separations step        operating at greater than 0.5 MPa and at less than −10° C. to        separate hydrogen therefrom,    -   iv) passing a portion of the first liquid stream to a second        separations step operating at less than 0.4 MPa and at less than        −10° C. to separate hydrogen therefrom.

The vapour comprising hydrogen may be the second stream previouslydescribed (where a contactor/fractionator combination is not present) ormay be the vapour from a contactor/fractionator combination where suchis used.

Steps (i), (ii) and (iii) are all performed at a pressure of at least0.5 MPa, usually at least 0.7 MPa, and preferably of at least 0.8 MPa.Thus, said steps may all be considered part of the high pressurerecovery system.

In contrast, step (iv) is operated at a relatively low pressure of lessthan 0.4 MPa, preferably less than 0.3 MPa, for example about 0.2 MPa.Preferably said step is integrated into the low pressure recovery systemsuch that all of some of said portion of the first liquid stream may berecovered and recycled to the reactor.

The preferred features of first stream, the feedstream and thecontacting of step (i) are as described previously. Thus, in thepreferred embodiments the contacting of the first stream with thefeedstream, which is preferably the effluent from a preceding reactor,results in an increase in hydrogen in the vapour comprising hydrogencompared to the first stream.

Step (ii) preferably comprises cooling the vapour to at least partiallycondense liquid therefrom, and separation of said third stream and firstliquid stream. This cooling is preferably achieved using a coolingmedium at above 0° C., most preferably using cooling water. By this stepthe majority of the diluent and unreacted monomers and comonomers arecondensed, leaving mainly “lights” such as hydrogen and methane,although also some monomer, in the vapour phase.

The separation may be performed in any suitable vessel, for example avapour/liquid separations drum.

In step (iii) said third stream is passed to a first separations stepoperating at greater than 0.5 MPa and at less than −10° C. to separatehydrogen therefrom. Due to the cooling and separation in step (ii) thethird vapour is only a relatively small portion of the vapour and thus amuch lower volume of material need be cooled to the low temperature ofthis step. The pressure in step (iii) is preferably approximately 0.8MPa. The temperature in step (iii) is preferably less than −20° C., morepreferably less than −30° C., and most preferably about −35° C.

At such low temperatures hydrogen is efficiently separated from monomerand other components in the third vapour which can be recycled. In step(iv) a portion of the first liquid stream is passed to a secondseparations step operating at less than 0.4 MPa and at less than −10° C.to separate hydrogen therefrom.

It has been found that taken a portion of the first liquid stream,cooling and depressurising for a second separations step results inefficient further recovery of hydrogen from the process.

The portion of the first liquid stream which is cooled and depressurisedfor a second separations step usually comprises at least 10% by weightof the first liquid stream. preferably at least 20% by weight, such asbetween 20 and 40 wt % and most preferably 20 to 30 wt % of the firstliquid stream.

The temperature in step (iv) is preferably less than −20° C., morepreferably less than −30° C., and most preferably about −35° C.

At such low temperatures hydrogen is again efficiently separated frommonomer and other components in the portion of the first liquid stream.Remaining components of the stream can be recycled (after compression).

The present invention allows to obtain a polymerisation process whichhas a high efficiency for desired components of the final polymer, suchas monomer, but “low” efficiency for other components (such ashydrogen).

As used herein, “efficiency” is a measure of the amount of a particularmaterial which is fed and which is not purged from the process. Forexample, monomer efficiency is the amount of monomer fed which is notpurged.

The monomer efficiency is a measure of the amount of the monomer whichends up in the polymer product, and is determined from the amount offresh monomer fed to a process and the amount of monomer which ispurged. The monomer purge rate may be determined from the purge flow andthe concentration of monomer in the purge stream, which can be measuredby GC, for each purge stream present. The efficiency may be determinedinstantaneously, based on flow rate measurements at a particular time,but preferably is determined over a period of time, for example based onaveraged instantaneous measurements or on total amounts fed and purgeddetermined over a period of at least several hours, as this generallygives a more accurate measurement. The monomer efficiency is determinedby subtracting the amount purged from the amount fed, and then dividingthe result by the amount fed. This answer is multiplied by 100 to givethe efficiency as a percentage.

The process of the present invention is able to provide a monomerefficiency in excess of 99.5%, for example of 99.6% and above, and mostpreferably of 99.7% and above.

It is worth noting that, whilst monomer efficiencies of polymerisationprocesses are generally very high (above 99%), at the scale ofcommercial polymerisation processes even what appear as relatively minorincreases in efficiency can result in significant cost savings, as wellas significant reductions in hydrocarbon emissions or combustionproducts from hydrocarbon emissions (when flared). For example, in aprocess producing 50 tonnes/hour of polymer, an increase in monomerefficiency by only 0.1% is still a reduction in monomer losses of 50kg/hour.

In contrast to a high monomer efficiency, it has been found that a lowhydrogen efficiency of a polymerisation process can be advantageous. Inparticular, hydrogen is more cost effectively flared than recycled andrecovered to the overall polymerisation process. An advantage ofrelatively low hydrogen efficiencies is that other impurities which canbe present in fresh hydrogen feeds, such as methane and CO, are alsoefficiently purged from the system via the purge streams, andpurification of fresh hydrogen feed via PSA can be avoided.

The present invention can result in a polymerisation process whichpreferably has a hydrogen efficiency, measured of the amount of the fedhydrogen which is not purged of 80% or less, preferably of 70% or less,and most preferably of 60% or less.

The hydrogen efficiency may be determined in a similar manner to themonomer efficiency, and in particular by determining the amount ofhydrogen purged from the purge flow and the hydrogen concentration inthe purge stream, which can be measured by GC, for each purge streampresent and comparing this to the amount of hydrogen fed to the process.

EXAMPLE

Ethylene is polymerised in two slurry loop reactors in series to producea bimodal polyethylene with a density of 948 kg/m³ and a Melt Index(MI₅) of 0.31.

In the first reactor ethylene is polymerised in the substantial absenceof comonomer, but in the presence of hydrogen and with isobutane asdiluent. Polymer from the first reactor is passed to a second reactorwherein further ethylene is polymerised in the presence of 1-hexene ascomonomer and the substantial absence of hydrogen, again in the presenceof isobutane.

Polymer slurry is withdrawn from the first reactor and passed to acontacting vessel in the form of a stirred tank.

Polymer slurry is recovered from the base of the contacting vessel andpassed to the second reactor.

Polymer slurry is withdrawn from the second reactor and passed via aslurry heater, in which the liquid components of the slurry arevaporised to a flash tank at a pressure of 0.85 MPa.

Polymer solids are withdrawn from the flash tank for further processing.The vapour recovered from the flash tank is passed, without furthertreatment, as the first vapour to the contacting vessel where it iscontacted with the slurry withdrawn from the first reactor. Thecontacting in the contacting vessel takes place at a pressure of 0.85MPa.

Vapour is withdrawn from the top of the contacting vessel (secondvapour) and passed to a fractionator in which it is contacted with areflux stream. Vapour recovered overhead is cooled and condensed. Aportion is utilised as the reflux stream to the fractionator. Themajority of the remainder is cycled to the first reactor.

Liquids recovered from the base of the fractionator are returned to thecontacting vessel.

The combined “contacting vessel/fractionator” is herein referred to as a“fractionation system”. The first vapour is passed to the contactingvessel. The stream comprises predominantly isobutane, but alsoapproximately 2700 kg/hr of 1-hexene and smaller quantities of hexane,ethane, ethylene and hydrogen.

The slurry from the first reactor is also passed to the contactingvessel. The slurry liquid comprises predominantly isobutane with smallerquantities of ethane and ethylene. The stream is substantially free of1-hexene but comprises approximately 13 kg/h of hydrogen.

The vapour recovered from the contacting vessel (second vapour stream)is substantially reduced in 1-hexene and hexane flow compared to thefirst vapour stream, but comprises substantially all of the hydrogen fedto the contacting vessel. The vapour from the contacting vessel ispassed to the fractionator in which further separation occurs. Theliquid recovered from the base and recycled to the contacting vesselcomprises essentially all of the 1-hexene and hexane in the secondvapour stream. The vapour recovered from the fractionator issubstantially free of 1-hexene and hexane, but comprises substantiallyall of the hydrogen fed to the fractionator, corresponding to 99.5% ofall hydrogen fed to the fractionation system in total).

The slurry recovered from the contacting vessel is, in contrast,substantially free of Hydrogen, but comprises essentially all of the1-hexene and hexane fed to the fractionation system.

It can be seen that the contacting vessel efficiently separates hydrogenfrom the slurry from the first reactor into the second vapour stream(the majority of which is cycled to the first reactor, where hydrogen isdesired), thereby also removing it from the second slurry (which issubsequently passed to the second reactor where hydrogen is notdesired). At the same time 1-hexene is separated from the vapour streamin the contacting vessel.

Further, by use of a fractionator in combination with the contactingvessel 1-hexene is essentially completely separated from the firstvapour stream and into the second liquid (which is cycled to the secondreactor, where the 1-hexene is required.)

Other “light” components, such as nitrogen, methane, ethane, ethyleneand propane are also preferentially separated into the vapour streamexiting the fractionation system. In particular, approximately 99% ofnitrogen, 98% of methane, 95% of each of ethane and ethylene, and 85% ofpropane end up in the vapour stream exiting the fractionation system(compared to “total” of each component fed to the fractionation system).

This stream is cooled to 35° C. (at 0.8 MPa) and passed to aliquid/vapour separation drum. Approximately 3500 kg/h of vapour isrecovered and further cooled to −35° C. (still at 0.8 MPa) to separate astream comprising 12 kg/h of hydrogen, which is flared. This stream alsocomprises methane, nitrogen, ethane and propane.

The majority of the liquid from the liquid/vapour separation drum isrecycled to the first reactor. A portion of the liquid from the liquidvapour separation drum, comprising principally isobutane, but alsoquantities of ethylene, ethane, propane and other components, includingapproximately 0.5 kg/h of hydrogen, is however cooled to −35° C. andlet-down in pressure to 0.2 MPa in the low pressure separations system.From the low pressure separations system there is separated and passedto flare a stream comprising 0.5 kg/h of hydrogen. Although this streamcomprises quantities of isobutane, methane, nitrogen, ethane andpropane, the majority (99.5 wt %) of the isobutane in the stream passedto the low pressure separations step is recovered for recycle (aftercompression) rather than flared.

1-15. (canceled)
 16. A process for the polymerisation of monomer in areactor, which process comprises contacting a first stream comprisingvapour derived from the effluent withdrawn from the reactor with a feedstream to the reactor, said first stream comprising the majority of thevaporised components of the liquid medium in the effluent withdrawn fromthe reactor.
 17. A process as claimed in claim 16 wherein the feedstream is in a liquid form.
 18. A process according to claim 16 whichcomprises first and second reactors connected in series and the feedstream is a feed stream to the second reactor, and preferably whereinthe feed stream comprises effluent derived from the first reactor.
 19. Aprocess according to claim 16 wherein the first stream comprises diluentand monomer, and may also comprise comonomers, hydrogen and impurities,such as alkanes and nitrogen.
 20. A process according to claim 16wherein the first stream is brought into contact with the feedstream tothe reactor at a pressure of 0.5 MPa or above and/or wherein the firststream is separated from the polymer withdrawn from the reactor at apressure of 0.5 MPa or above.
 21. A process according to claim 16wherein the effluent withdrawn from the reactor is a slurry of polymersolids in a liquid medium comprising diluent, and wherein the firststream is derived from the effluent by flashing all or a portion of theliquid medium to form a vapour, and separating this from the polymersolids, and preferably wherein the first stream is a majority of theseparated stream.
 22. A process according to claim 16 wherein a portionof the first stream is condensed prior to contact with the feedstream.23. A process according to claim 16 wherein the majority of the firststream is in vapour form when brought into contact with the feed stream,and preferably wherein at least 80% by weight of the first stream is invapour form when brought into contact with the feed stream.
 24. Aprocess according to claim 16 wherein the first stream is passed fromits source to the contacting step without any compression and/or whereinthe first stream is passed from its source to the contacting stepwithout passing through any filters and/or wherein the first stream ispassed directly from its source to the contacting step with the feedstream.
 25. A process according to claim 16 wherein the first streamcomprises entrained solids which are preferentially entrained into thefeed stream.
 26. A process according to claim 16 which is a process forthe production of bimodal polymer products.
 27. A process according toclaim 16 wherein the contacting takes place in a contactor which isconfigured in association with a fractionator, and preferably whereinthe vapour, or at least a portion thereof, recovered from the contactoror the contactor/fractionator combination is passed to one or more stepswhich include removal of inert components, especially inert lightcomponents and/or include removal of heavy components prior to recycle.28. A process according to claim 16 wherein the process has a monomerefficiency in excess of 99.5% and/or has an hydrogen efficiency of 80%or less.
 29. A process for the polymerisation of monomer in at least tworeactors operated in series, which process comprises contacting a firststream comprising vapour derived from the effluent withdrawn from thesecond reactor with a feed stream to the second reactor to produce asecond stream comprising vapour which is passed to further processingand a third stream which is passed to the second reactor.
 30. A processas claimed in claim 29 wherein the feed stream is in a liquid form. 31.A process as claimed in claim 29 wherein the feed stream compriseseffluent derived from the first reactor.
 32. A process according toclaim 29 wherein the first stream comprises diluent and monomer, and mayalso comprise comonomers, hydrogen and impurities, such as alkanes andnitrogen.
 33. A process according to claim 29 wherein the first streamis brought into contact with the feedstream to the second reactor at apressure of 0.5 MPa or above and/or wherein the first stream isseparated from the polymer withdrawn from the second reactor at apressure of 0.5 MPa or above.
 34. A process according to claim 29wherein the effluent withdrawn from the second reactor is a slurry ofpolymer solids in a liquid medium comprising diluent, and wherein thefirst stream is derived from the effluent by flashing all or a portionof the liquid medium to form a vapour, and separating this from thepolymer solids, and preferably wherein the first stream is a majority ofthe separated stream.
 35. A process according to claim 29 wherein aportion of the first stream is condensed prior to contact with thefeedstream.
 36. A process according to claim 29 wherein the majority ofthe first stream is in vapour form when brought into contact with thefeed stream, and preferably wherein at least 80% by weight of the firststream is in vapour form when brought into contact with the feed stream.37. A process according to claim 29 wherein the first stream is passedfrom its source to the contacting step without any compression and/orwherein the first stream is passed from its source to the contactingstep without passing through any filters and/or wherein the first streamis passed directly from its source to the contacting step with the feedstream.
 38. A process according to claim 29 wherein the third stream isin a liquid form and preferably comprises a slurry of polymer solids ina liquid medium.
 39. A process according to claim 29 wherein the firststream comprises entrained solids which are preferentially entrainedinto the third stream.
 40. A process according to claim 29 which is aprocess for the production of bimodal polymer products.
 41. A processaccording to claim 29 wherein the contacting takes place in a contactorwhich is configured in association with a fractionator, and preferablywherein the vapour, or at least a portion thereof, recovered from thecontactor or the contactor/fractionator combination is passed to one ormore steps which include removal of inert components, especially inertlight components and/or include removal of heavy components prior torecycle.
 42. A process according to claim 29 wherein the process has amonomer efficiency in excess of 99.5% and/or has an hydrogen efficiencyof 80% or less.